You may ultimately find that your grip contributed to the Mag Drop issue. 30 rounds thru an M&P40 does not yield very much time to become familiar with it. I have a 40 and 9, both of early production, sn. MPAxxxx and MPBxxxx. The first day with my first M&P I bumped the release a couple of times and thought I had a problem. 10,000 rounds later I have concluded the problem was me. The standard M&P release is considerably easier to operate than several other types I have used which is a plus when trying to reload quickly.
I hope, when you get it back, you give it a bit more of a test than 2 magazines of ammo.
Re-reading allowed me to catch my error in thinking you only shot 30 rounds before sending it back. but I still think the gun may be ok. Let us know what results you have